1. Field of the Invention
The present invention relates to the field of communications, and in particular, to line drivers.
2. Related Art
A local-area network ("LAN") is a communication system that enables personal computers, work stations, file servers, repeaters, data terminal equipment ("DTE"), and other such information processing equipment located within a limited geographical area such as an office, a building, or a cluster of buildings to electronically transfer information among one another. Each piece of information processing equipment in the LAN communicates with other information processing equipment in the LAN by following a fixed protocol (or standard) which defines the network operation. Information processing equipment made by different suppliers can thus be readily incorporated into the LAN.
The ISO Open Systems Interconnection Basic Reference Model defines a seven-layer model for data communication in a LAN. The lowest layer in the model is the physical layer which consists of modules that specify (a) the physical media which interconnects the network nodes and over which data is to be electronically transmitted, (b) the manner in which the network nodes interface to the physical transmission media, (c) the process for transferring data over the physical media, and (d) the protocol of the data stream.
IEEE Standard 802.3, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, is one of the most widely used standards for the physical layer. Commonly referred to as Ethernet, IEEE Standard 802.3 deals with transferring data over twisted-pair cables or co-axial cables. The 10 Base-T protocol of IEEE Standard 802.3 prescribes a rate of 10 megabits/second ("Mbps") for transferring data over twisted-pair cables.
The constant need to transfer more information faster, accompanied by increases in data processing capability, necessitated an expansion to data transfer rates considerably higher than the 10-Mbps rate prescribed by the 10 Base-T protocol. As a consequence, a protocol referred to as 100 Base-T was developed for extending IEEE Standard 802.3 to accommodate data moving at an effective transfer rate of 100 Mbps through twisted-pair cables. Under the 100 Base-T protocol, certain control bits are incorporated into the data before it is placed on a twisted-pair cable. The result is that the data and control signals actually move through a twisted-pair cable at 125 Mbps.
In expanding IEEE Standard 802.3 to the 100 Base-T protocol, there are various situations in which it is desirable that the transmitter be capable of using one driver to transmit data at both the 100 Base-T rate and the lower 10 Base-T rate. Accordingly, it is preferable to use a line driver capable of driving both 10 Base-T and 100 Base-T signaling.
In particular, one set of information processing equipment should be capable of driving data moving at the 10 Mbps ("Meg") rate or the 100 Meg rate without having to make any adjustments when the data transfer rate changes from 10 Meg to 100 Meg and vice versa.
FIG. 1 illustrates the data transmit path 100 of communication in the LAN operating in 100 Base-T. During data transmission, a communication unit operating on the LAN, such as a computer 117, generates a data signal T1 which is converted into differential form for transmission on the twisted pair cable 103. For 10 Base-T transmission, this data signal T1 is Manchester coded 101 to reduce electromagnetic interference and to produce square wave pulses. These waves then go through a waveshaping filter to generate filtered differential data signals T1+/-.
In this description a pair of differential signals means two signals whose current waveforms are out of phase with one another. The individual signals of a pair of differential signals are indicated by reference symbols respectively ending with "+" and "-" notation-e.g., S+ and S-. The composite notation "+/-" is employed to indicate both differential signals using a single reference symbol-e.g., S+/-.
For 100 Meg transmission, scrambler 119 scrambles data signal T1 and converts data signal T1 to differential format. Encoder 121 MLT-3 codes the data signal to generate trinary differential signals T2+/-. A 10 Meg amplifier signal driver 107 and a 100 meg amplifier signal driver 109 take these differential signals T1+/- and T2+/-, respectively, and generate voltage-sourced differential signals T10+/- and T20+/- respectively, to drive a primary load 105 and to transmit them on twisted pair cable 103.
Transformer 111 has a primary winding 111A and a secondary winding 111B which isolate the twisted-pair cable 103 from the circuitry producing the transmit signals. Primary winding 111A terminates at a primary load 105 and secondary winding 111B terminates at a secondary load 113. Secondary load 113 couples to a connecting unit 115, which couples to twisted-pair cable 103. Primary winding 111A couples to a resistive load 105. It is across this resistive load 105 that either sine wave 10 Base-T signaling or MLT-3 100 Base-T signaling must be created.
Recently, current driven amplifiers have been used to drive both the 10 Base-T and the 100 Base-T signalling. Now that it also has become necessary for many commercial integrated circuits to operate at less than the conventional 5 volt power supply voltage, such as 2.5 volts, these line driver circuits must operate over a power supply range from over 5 volts down to 2.5 volts and less. However, although the supply voltage dropped from 5 volts to 2.5 volts, the 10 Base-T signaling mode still requires a 5-volt peak-to-peak output voltage.
FIG. 2 illustrates a conventional line driver 200 capable of driving both 10Base-T and 100Base-T signaling. Typically, as the circuits move down to lower supply voltages, there is not enough headroom for current sources I21-I23 to operate. Thus, transformer T2 operates in a 1:2 voltage step-up mode with respect to the differential data signals, to boost output voltage Vout to 5 volts peak-to-peak. The difference in voltage between the output nodes V+, V- on the output terminals form the pair of differential signals.
Line driver circuit 200 includes a resistive load RL coupled between output nodes V+, V- and to the primary winding of transformer T2. The secondary winding of transformer T2 couples to a termination resistor R2. Typically, this termination resistor is approximately 100 ohms as required by the Institute of Electrical and Electronics Engineers (IEEE). Line driver circuit 200 also includes a 10Base-T sub-circuit 201 to generate a 5 volt peak-to-peak output signal Vout across resistive load RL, and a 100Base-T sub-circuit 202 to generate a 2 volt peak-to-peak output signal Vout across resistive load RL. The 10Base-T sub-circuit 201 includes switches SW21, SW22, and current sources I21, I22 each of which provide 100 milliamperes (mA) of current. The 100Base-T sub-circuit 202 includes switches SW23-SW26 and current source I23 which provides 40 mA of current. The six switches SW21-SW26 are controlled by input signals IN21-IN26, respectively, and direct current through load resistor RL as indicated by arrows A and B. Typically, these input signals IN21, IN22 are rail-to-rail voltage swings, and input signal IN21 is the inverse of input signal IN22.
In 10Base-T operation, input signals IN27, IN28 are half-wave rectified signals which are 180 degrees out of phase from one another. These signals are applied to current sources I21, I22, respectively, such that only one of the two current sources is active at a time. In addition, input signals IN21, IN22 are applied to switches SW21, SW22 such that one of the switches SW21, SW22 closes and the other opens to steer current through resistive load RL thereby generating a voltage across resistive load RL. During this time, input signal IN29 is applied to current source I23 to keep this current source turned off so 100Base-T sub-circuit 202 is inactive.
In 10Base-T operation, to steer current through resistive load RL in the direction indicated by arrow A, input signals IN21, IN22, IN27 and IN28 are applied such that switch SW21 opens and switch SW22 closes, and current source I22 turns off and current source I21 turns on. In this way, current flows through switch SW22, through transformer T2 and resistive load RL to generate half of the differential output signal Vout. Conversely, to steer current through resistive load RL in the direction indicated by arrow B, input signals IN21, IN22, IN27 and IN28 are applied such that switch SW21 closes and switch SW22 opens, and current source I21 turns off while current source I22 turns on. In this way, current flows through switch SW21, through transformer T2 and resistive load RL, to generate the other half of the differential output signal Vout. As a result, a full differential output voltage swing can be achieved.
On the other hand, in 100Base-T operation, input signal IN29 turns on current source I23. Input signals IN23-IN26 are applied such that two of the four switches SW23-SW26 close at a time to steer the 40 mA current from constant current source I23 and generate a voltage across resistive load RL. During this time 10Base-T sub-circuit 201 is inactive. Input signals IN23-IN26 are also rail to rail voltage swings.
To steer the 40 mA current through resistive load RL in the direction indicated by arrow A, input signals IN23 and IN26 are applied such that switches SW23 and SW26 close at the same time to conduct the current. When switches SW23 and SW26 close, switches SW24 and SW25 open based on input signals IN24, IN25. Thus, output node V- is pulled up toward voltage supply VDD whereas output node V+ is pulled down towards circuit ground.
Subsequently, switches SW23 and SW26 open, causing output voltage Vout to move toward circuit ground. Then, switches SW24 and SW25 close, while switches SW23 and SW26 open to steer current through resistive load RL in the direction indicated by arrow B. Thus, output node V- is pulled down whereas output node V+ is pulled up toward VDD. As a result, the difference in voltages at output nodes V+, V-, provides the MLT-3 output voltage waveformn needed to drive line driver 200 in 100Base-T operation.
Although line driver 200 can operate with a 2.5 supply voltage, line driver 200 requires 1:2 transformer T2. This is because it is impossible to have any headroom for the current source or voltage drop in the switches while driving a 2.5 volt signal across the load with a supply voltage of 2.5 volts or less. These 1:2 transformers are typically more expensive and have lesser quality than 1:1 transformers. In addition, line driver circuit 200 produces a large common mode component Vc, where ##EQU1##
where V+ is the voltage at output node V+, and V- is the voltage at output node V-. It is desirable that common mode component Vc be zero, indicating that the voltage signal at output node V+ is exactly out of phase with the voltage signal at output node V-. When this occurs, the signals are truly differential. However, typically the signals are not truly differential and thus, the common mode component Vc is not zero. This common mode component Vc radiates and causes electromagnetic interference ("EMI") on the transmission line. Although such common mode component Vc may be more tolerable in 10Base-T operation due to the lower 10 MHz frequency, such common mode component Vc cannot be tolerated in 100Base-T operation having a higher 125 MHz frequency.
One conventional line driver circuit 300 that operates in both 10Base-T and 100Base-T signaling and overcomes some of the above problems, is illustrated in FIG. 3. This conventional line driver circuit 300 is more advantageous than conventional line driver circuit 200 because it utilizes a 1:1 transformer T3 and can still operate with a 2.5 volt supply voltage. In this circuit 300, switches SW21, SW22 from line driver circuit 200 are eliminated, and switches SW23 and SW24 from conventional line driver circuit 200 are replaced by 50 ohm resistors R31, R32, respectively. These resistors R31, R32 provide termination for transformer T3 as well as provide two paths to pull current through line driver circuit 300.
Operation of line driver circuit 300 is similar to that of line driver circuit 200. In 10Base-T operation, half-wave rectified signals input signals IN27, IN28 are applied to 100 mA current sources I21, I22, respectively, to such that they alternate between being active and inactive to steer the 100 mA current through resistive load RL in the direction indicated by arrows A and B. For example, when current source I21 is active, current flows to the primary winding of transformer T3 which induces current to flow through resistor RL in the direction indicated by arrow A. In 100Base-T operation, input signals IN25, IN26 control switches SW25, SW26 to open and close to steer the 40 mA current through resistive load RL in the direction indicated by arrows A and B.
As illustrated in FIG. 3, the center tap CT of transformer T3 couples to voltage supply VDD. In this way, common mode voltage Vc is forced to equal supply voltage VDD. Since supply voltage VDD is a constant at 2.5 V, common mode voltage Vc is fixed at 2.5 Volts. Using equation (1), in order to obtain the necessary headroom for line driver circuit 300 to drive 5 volts peak-to-peak with a 2.5 volt supply voltage, output nodes V-,V+ of line driver circuit 300 swing above and below supply voltage VDD.
In operation, since input signals IN27, IN28 are 180 degrees out of phase from one another, when current source I22 is active current source I21 is inactive and vice versa. When current source I21 is active it pulls the voltage at output node V+ down and consequently the voltage at output node V- goes up due to the transformer T3 action. The maximum single-ended output voltage swing Vmax is ##EQU2##
where Vp-p is the peak-to-peak voltage across transformer T3. Substituting 5 volts for the desired peak-to-peak voltage Vp-p and 2.5 volts for VDD, ##EQU3##
Since maximum single-ended output voltage swing Vmax is 3.75 (Vmax=1.25+2.5), output voltage Vout swings above common mode voltage Vc, which is supply voltage VDD, by approximately 1.25 volts.
Since output voltage Vout exceeds supply voltage VDD, the devices which make up current sources I21-I23, typically Complementary Metal Oxide Semiconductor ("CMOS") devices, tend to breakdown. These CMOS devices are only intended to handle a maximum supply voltage VDD, which is shown in FIG. 2 as 2.5 volts. When output voltage Vout of line driver circuit 300 goes above this voltage level, which can be as high as approximately 4 volts (2.5 V+1.25 V), the CMOS devices fail due to their inability to withstand the high voltage. Thus, special circuits and special processes are needed for line driver circuit 300 to operate properly.
Since output voltage Vout swings above and below supply voltage VDD, this line river circuit 300 eliminates the headroom limitations of conventional line driver circuit 200. This line driver circuit 300 also eliminates common mode voltage Vc radiation by fixing it at supply voltage VDD. However, the disadvantages of this circuit are the special process and/or circuit requirements needed to prevent breakdown of the components of the circuit. Certain processes utilize dual gate oxide transistors such as, CMOS devices, which are capable of handling higher voltages at the cost of lower performance, such as decreased speed or increased area. Optionally, certain processes may allow particular operating conditions such that a regular CMOS device can withstand higher voltages for shorter periods of time. Usually, these restrictions are quite limiting and the resulting circuit configurations are large in area and poorer in performance.
Another disadvantage of line driver circuit 300 compared to line driver circuit 200 is that twice the current must be driven into the load and the termination network to obtain the required voltage swing. Therefore, the circuit may need to be configured for class B operation to drive both 10Base-T and 100Base-T signaling to save power.
Thus, a need exists for a cost effective and high quality line driver circuit capable of driving both 10Base-T and 100Base-T signalling using a low voltage power supply.